1. Field of the Invention
The invention relates to a process of making such a printing screen and a method of screen printing on hard non-absorbent materials, such as glass, to provide a pattern with selected areas of variable ink thickness.
2. Summary of Related Art
Screen printing is an important process in making automotive glass and other glass components, for making printed circuit boards, and for a number of other applications where a pattern is printed on a hard substrate surface. In the glass manufacturing process, screen printing is used to print the electrically conducting networks for heater arrays on rear windows and backlights. Such heater arrays on the glass generally comprise an array of heating conductors extending across the window between bus bars adjacent the lateral edges of the glass. In addition to printing specific patterns, such as electrical conductors, screen printing is also used to apply bands of paint for shading and privacy obscuration purposes.
The heater array may be printed on one surface of a piece of glass before the glass is heated for bending and hardening. A ceramic ink/paint with an electrical conductor, such as silver, is used to form the heater pattern during the screening process. The wet ceramic ink is spread on the screen during production use. A squeegee blade is pressed across the screen to force ink through the screen in any of the apertures in the screen coating. A return run of the squeegee blade back across the screen forces the ink onto the surface of the glass.
In automotive glass printing applications, it is desirable to vary the thickness of the printed ink pattern at one or more locations on the glass. A thicker ink pattern will increase the conductance of parts of the pattern such as bus bars, to provide a thicker connector to solder leads to a bus bar, or for a number of other purposes in connection with automotive glass and other applications.
In addition to electrically conductive networks, screen printing is also used to provide painted ceramic masks for tinted windows and obscuration areas along the edge of a window. Color masking with a graduation area along an edge of a window glass of an automobile is often performed to hide mechanical components and structure along the edge of the window. Screen printing may also be used to provide a mask or dot pattern used to tint windows for sun screening purposes.
A number of printing processes, including the glass manufacturing, are switching from leaded paint to no-lead paint for environmental reasons. When using no-lead paints for tinted windows and obscuration areas, heavier deposits of paint are required to achieve the same opacity.
Special problems occur in printing glass in areas of the window where the electrically conductive networks extend into tinted or obscuration areas. The painted ceramic mask is applied and dried around the edge of the window using known screen printing techniques, which creates a generally vertical surface at the edge of the paint. A second screen printing application is used to apply the electrical bus bars and heater grid. The conductive grid lines pass over the edge of the paint band or the edge of the dot patterns forming the paint band. The conductive grid line loses continuity at the edge of the paint band and an open circuit condition occurs in the heater grid.
Various printing screen structures and manufacturing methods are known in the art. The typical printing screen for automotive glass starts by preparing the original pattern as a positive film, and then transferring the positive image on a positive film to a full-size photosensitive transparency. A screen is prepared by coating the screen with a photosensitive emulsion and placing the transparency on the screen. A light source on the transparency side of the screen is switched on for the required exposure time. The screen is separated from the transparency and the non-exposed areas of emulsion are washed away to form the desired pattern in the screen.
The thickness of the layer of paint/ink which is deposited on the glass or other substrate depends on a number of factors, including size of the mesh in the screen, the type of paint/ink, the thickness of the hardened emulsion formed in the screen, the pattern of the emulsion in the deposit areas, and other similar factors. The thickness of the emulsion depends on the thickness of wet emulsion applied to the screen and the photo cure time to harden the emulsion. In many cases, the emulsion must be applied in several steps to harden the emulsion to the desired thickness.
A number of U.S. patents have discussed a method for achieving the desired patterned ink layer of variable thickness. U.S. Pat. Nos. 3,851,581 and 3,852,564 to Baum et al disclose a method of making a silk screen and the manufacture of electrically heated windows. The references teach a screen having a uniform thickness and that the uniform thickness may be varied by depositing successive layers of emulsion. The layers of emulsion will not harden properly if the emulsion is too thick. Multiple layers of emulsion are used to build up the screen, with each layer being dried and hardened before applying a subsequent layer. The same transparency is used for each layer of emulsion and it is essential that the transparency be positioned exactly to the pattern.
U.S. Pat. No. 4,958,560 to Collins teaches a method of screen printing a patterned ink layer using a screen with a patterned coating on its surface facing the substrate. The reference discloses that the thickness of a patterned ink layer screen printed on a glass surface can be increased in selected areas by providing local support between the screen surface and the glass in the selected areas. One of the methods for providing the local support between the glass and the screen to increase ink retention in those areas is an array of spots of coating on the screen. Another method of support is to provide extra patches of screen in the selected areas.
The automatic drawing apparatus disclosed in U.S. Pat. No. 4,975,860 to Kitaya et al may be used to develop dot patterns. The design of the dot patterns may be evaluated and modifications in the pattern may be completed on the apparatus. The patterns used to form the array of dots for a screen process may be generated in a cost efficient manner.
U.S. Pat. No. 4,379,737 to Mearig and U.S. Pat. No. 4,791,006 to Galvagni et al disclose additional methods for providing variable thickness build up. Mearig teaches a rotary printing screen having a built area to achieve additional thickness. Galvagni et al method is a high accuracy method for forming conductive paths of varying thickness.
In the copending application Ser. No. 08/057,680, the variable thickness of paint deposits was achieved by forming a thicker layer of emulsion on the screen in a dot pattern. The thicker dot configuration was then used to form a thicker layer of paint in the designated areas.
The screens and the method for screen printing referred to in the above references suffer from a number of disadvantages. Glass manufacturers are still working to develop a fast, convenient, and cost effective means for screen printing which will provide the variable thickness ink in the desired patterns. Similar methods could also be applied to a number of other screen printing applications, such as printed circuit boards.
In the glass industry, there is a need for a low cost screen which will selectively deviate from the standard deposit thickness of the ink or paint to form both thinner deposit areas and thicker deposit areas in the same screen. The extra thickness of ink provides a thicker conductor for weld points and other similar applications. The thinner thickness of ink may be used at the edges of the paint band edge to reduce breaks in the grid lines. The screen must be rugged for use on a sufficient number of production pieces of glass without the disintegration of the hardened emulsion.
In a dot pattern, the dots in many screen designs have a tendency to crumble when used in production. The blade delivering the ceramic ink to the screen places a downward force on the screen. If the hardened emulsion dots crumble, the screen becomes ineffective for furnishing increased ink to the glass and emulsion pieces may cause quality problems in printing the pattern on the glass, which may cause the complete piece of glass to be rejected.